Structural Ledges Research Articles

The Effect of β Stabilizers on the Structure and Energy of α/β Interfaces in Titanium Alloys

The structure and energy associated with interfaces between the BCC and HCP lattices (β and α phase, respectively) in titanium alloys with commonly used β stabilizers were analyzed. For this purpose, the crystallographic structure of the matching facets of broad, side and end faces was described using misfit dislocations and structural ledges which compensate the mismatch in atomic spacing of the α and β phases. The effect of the β/α transformation temperature due to various concentration of β stabilizers on periodicity of misfit dislocations and structural ledges was estimated. The van der Merwe approach was used to calculate energy of different matching facets. An increase in the percentage of β-stabilizing elements was found to result in a decrease in the lattice-parameter ratio (aβ/aα) and an increase in the energy of all faces. The dependence of the interface energy on the aβ/aα ratio was for the first time quantified, and insight into the preferred shape of α-phase precipitates was obtained.

Irrational crystallography of the ⟨1 1 [formula omitted] 0⟩Mg Mg2Sn precipitates in an aged Mg-Sn-Mn alloy

Mg2Sn precipitates growing along ⟨1 1 2¯ 0⟩Mg are the dominant strengthening phase in aged Mg-Sn-based alloys. They were reported to exhibit an ‘lath’ morphology with an rational orientation relationship (OR) of (1 1 0)β//(0 0 0 1)α, [0 0 1]β//[1 1 2¯ 0]α in previous studies. In this work, the ⟨1 1 2¯ 0⟩Mg Mg2Sn in an aged Mg-Sn-Mn alloy was found to actually have a long-lenticular-like morphology. The lenticular Mg2Sn contains a midrib separating two pairs of broad interfaces and one pair of side interfaces. Both of the broad interfaces deviate from the (0 0 0 2)Mg plane by about 8°. The distinct morphology is associated with an near-rational OR of (1 1 0)β//(0 0 0 1)α, and [0 0 1]β 0.6° from [1 1 2¯ 0]α. Using the secondary constrained coincidence lattice model, the OR was revealed to make good lattice matching realized across two pairs the interfaces. The detailed interfacial structure of the two interfaces were both predicted to contain an arrays of periodically distributed structural ledges, and each ledge is associated with a misfit-compensating dislocation. These defects configurations show reasonable well agreement with HRTEM results.

Observation of heterostructure epitaxy of Pt-doped Ni-monosilicide on Si(001)

The objective of this study was to determine detailed microstructure of a Ni1–xPtxSi film formed via a melting/quenching process using high temperature laser annealing on a Si(001) substrate. The orthorhombic Ni1–xPtxSi film was found to be able to epitaxially grow with a crystallographic relationship of Ni1–xPtxSi[010]//Si[110], Ni1–xPtxSi(400)//Si(33¯1), and Ni1–xPtxSi (104)//Si(004). Volume expansion of the Ni1–xPtxSi film due to Pt incorporation was mainly accommodated by an increase in only one direction nearly parallel to the film surface (lattice parameter a). This was explained by the minimum coherent strain at the Ni1–xPtxSi (104)/Si(004) interface with an epitaxial growth tendency. Atomic-scale scanning transmission electron microscopy analyses revealed that the interface of Ni1–xPtxSi/Si had a repetitive atomic-step feature with energetically favorable Ni1–xPtxSi(004) terraces and (400) structural ledges that could increase the coherent area. By generating an array of misfit dislocations with an extra half plane of Ni1–xPtxSi(020), the elastic strain was further relieved.

Multiple Twinning and Stacking Faults in Silver Dendrites

Detailed defect structure of dendrite formation was studied in order to connect the mesoscopic with the atomistic structure. It was demonstrated that twinning and stacking fault formation play a central role in the growth of electrodeposited Ag dendrites. The broad faces of Ag dendrites and the main trunk growth direction were found to be (1̅11) and [1̅12̅], respectively. Dendrite branches also formed and grew from the main trunk parallel to the [121̅] and [2̅1̅1̅] crystallographic directions. Twins and stacking faults were found to reside on the {111} crystallographic planes, as expected for a face centered cubic (FCC) Ag crystal. Using electron back scattered diffraction (EBSD) we found two variants of in-plane 60° rotational twin domains in the (1̅11) broad dendrite surface plane. The intersections of twins and stacking faults with dendrite arm surfaces are perpendicular to the ⟨112⟩ arm growth directions. However, occasionally twins on the {111} planes parallel to the ⟨112⟩ arm growth directions were also observed. Although defect assisted dendrite growth is facilitated by twinning and stacking fault formation on {111} planes, the growth directions of the trunk and branches are not of the ⟨111⟩ type, but rather close to ⟨112⟩. The ⟨112⟩ growth directions are maintained by breaking dendrite facets into thermodynamically stable 111 and 200 steps and structural ledges of different length.

Formation and Characterization of Femtosecond-Laser-Induced Subcluster Segregated Nanoalloys

We report the first synthesis of subcluster segregated nanoalloys formed through the joining of immiscible metallic nanoparticles (NPs) using femtosecond (fs) laser irradiation. Immiscible alloy components consisting of Ag and Ni, and Ag and Fe, all in the form of NPs, were first deposited on a carbon film in vacuum by fs laser ablation from the parent metals. These samples of randomly distributed NPs were then irradiated with multiple fs laser pulses at a fluence of 1.5 mJ/cm2. Transmission electron microscopy (TEM) observations indicate that Ag and Ni as well as Ag and Fe NPs were successfully joined under these conditions. Energy dispersive X-ray (EDX) results show that no mixing layer exists at the interface. The nanostructure in the interface reveals the assumption of a specific angle between two matching planes on either side of the interface. Calculation of the lattice mismatch indicates that the system adjusts to this angle so as to reduce surface energy. Structural ledges were also formed at the ...

Predicting equilibrium shape of precipitates as function of coherency state

A general approach is proposed to predict the equilibrium shapes of precipitates in crystalline solids as function of size and coherency state. The model incorporates effects of interfacial defects such as misfit dislocations and structural ledges on transformation strain and on interfacial energy. Using α precipitation in α/β titanium alloys as an example, various possible equilibrium shapes of precipitates having different defect contents at interfaces are obtained by phase-field simulations. The simulation results agree with experimental observations in terms of both precipitate habit plane orientation and defect content at the interface. In combination with crystallographic theories of interfaces and experimental characterization of habit plane of finite precipitates, this approach has the ability to predict the coherency state (i.e. defect structures at interfaces) and equilibrium shape of finite precipitates.

Crystallography and migration mechanisms of planar interphase boundaries

A general and yet simple theory is developed to predict the orientation relationship, orientation, structure and migration mechanisms of planar interphase boundaries in phase transformations. In contrast to existing theories that are based essentially on matching of atoms or lattice sites within two parallel, closely-packed planes in the two lattices, the present theory involves consideration of matching of edges of closely-packed lattice planes. In terms of the present theory, planar interphase boundaries are defined by the Moiré plane resulting from the intersection between at least two sets of closely-packed lattice planes, and that such boundaries migrate in their normal directions by successive nucleation and lateral gliding, within the macroscopic interphase planes, of interfacial defects of Moiré ledges that have unique heights and displacement vectors. A brief comparison is provided between the present theory and approaches of phenomenological theory of martensite crystallography, invariant line, structural ledges and disconnections.

The cellular interlamellar and growth-front interphase boundaries in Cu-3 Wt pct Ti

Examination of the cellular colony interlamellar and growth-front interphase boundaries in Cu-3 wt pct Ti reveals an influence of crystallography at both of these interface types. Analysis of the interlamellar boundaries demonstrates that different arrangements of interphase misfit-compensating defects exist and combinations of misfit dislocations (MDs), structural ledges (SLs), or direction steps (DSs) were observed to dominate strain reduction between lamellae, even within the same colony. Detailed analysis also demonstrated that the actual interlamellar orientation relationship (OR) is (111) α ‖ (010) β with [-101] α ‖ [501] β , which is 0.28 deg in misorientation from the reported OR. The effect of crystallography was also apparent at the cellular growth front, as evidenced by the misfit-compensating structure observed with transmission electron microscopy (TEM) at the grain-boundary segments and the sharp faceting of all β precipitate-growth interfaces.

Comparison of interfacial structure-related mechanisms in diffusional and martensitic transformations

Some central problems in understanding the similarities of and the differences between ledgewise martensitic and ledgewise diffusional growth are examined. Martensitic growth can be described in terms of a lattic correspondence and a plane undistorted by the shear transformation. Diffusional growth can be similarly described in some cases but not in others. On the basis of the Sutton-Balluffi definitions of glissile and sessile boundaries, only misfit dislocations (on terraces or risers) or orthogonal sets of disconnections provide a truly sessile interface. When closely spaced structural ledges (now termed “structural disconnections”) are present during diffusional growth, they must have been glissile in the formation of a local equilibrium structure during the initial stages of growth. Once they are in local equilibrium and evenly spaced, however, they can only move synchronously because of their local strain interaction. Under these circumstances, extrinsic sources of growth ledges are required to move such interfaces in a diffusional manner. During martensitic growth, however, disconnections in the form of transformation dislocations can move freely in a synchronous manner. Also, on this basis, the apparent (undistorted) habit plane is generally useful in deducing the transformation mechanism during martensite formation, but is only occasionally so during diffusional growth, where only the terrace plane is generally useful.

Surface relief effects of α precipitates in a Ti–Mo alloy

The characteristics of the formation of surface relief associated with h.c.p. α precipitates in a Ti–Mo alloy were studied by atomic force microscopy coupled with electron back scattering diffraction and three-dimensional morphological observation. Tent- or inverse tent-shaped relief was formed with normal α plates (thick plates formed often with a ragged interface at smaller undercoolings) progressively to a lesser extent with decreasing temperature. It was replaced entirely by invariant plane strain (IPS) type relief as the plate morphology evolved to black plates (thin straight plates formed at larger undercoolings). In contrast to IPS relief the tilt angles of tent- and inverse tent-shaped relief were often considerably greater than expected from the motion of biatomic structural ledges. There seems to be a marked correlation of relief type with the surface orientation of matrix grains. A phenomenological explanation is presented for unduly large relief angles on the basis of previous theories.

HRTEM Observation and Atomic Modeling of α/β Interphase Boundary in a Ti-22V-4Al Alloy

In order to study interface boundary structures, a three dimensional Near-Coincidence Site lattice (Near-CSL) model based on Bollmann’s O-lattice method has been proposed. Previous studies have shown that intragranular α-phase precipitates in a Ti–22V–4Al alloy have a morphology of lath-like, with two well defined facet planes. A detailed TEM observation of the laths has been made in this study and the results correlated to the present analysis. Accommodation mechanism of misfit dislocations and the Burgers vector of the defects observed at the broad face and side facet plane of the intragranular α-phase precipitates have been determined. The structural ledges at the side facet, as predicted by the present analysis, is consistent with HRTEM observation. The Burgers vector of misfit dislocations at the broad face is in good agreement with that previously reported for a Zr–2.6Nb alloy, that has almost the same lattice parameter ratio as the Ti–22V–4Al alloy used in the present study. Although the Burgers vector of the defects on the broad face can be determined by HRTEM observation, the boundary plane is not well defined. However, it can be demonstrated that the interface boundary structure of the broad face can be rationalized from the results of both the Near-CSL analysis and the HRTEM observations.

Mechanism and crystallography of ferrite precipitation fromcementite in an Fe-Cr-Calloy during austenitization

Abstract The morphology of ferrite precipitates formed within cementite and the mechanism of their formation were examined by means of transmission electron microscopy in an Fe-2.6 wt% Cr-0.96 wt% C alloy after austenitizing in the range 800–950°C for various times. At the beginning of the austenitization process, the cementite particles dissolve rapidly by carbon diffusion in the austenite. The precipitation of ferrite occurs during the second stage of transformation when cementite dissolution is controlled by chromium diffusion in cementite or in the austenite matrix. The ferrite precipitates exhibit a lath-like shape elongated in the [010] cementite direction with the habit plane close to the (001) cementite. The cementite and ferrite were related to each other by two previously reported orientation relationships. In the case of the Pitsch-Petch orientation relationship, the habit plane of the ferrite lath is almost parallel to (152)f//(001)c and the ferrite-cementite interface is flat without any steps. The side facet of the lath is close to (2 15)f//(101)c, and the ferrite growth direction is close to [311]f//[010]c. In the case of the Bagaryatsky orientation relationship, the habit plane of the ferrite lath deviates several degrees from the broad face of (1 12)f//(001)c and structural ledges form at the ferrite-cementite interface, preserving coherency. The side facet of the lath is close to (121)f//(101)c and the ferrite growth direction is close to [111]c//[010]c. The results are discussed by assuming local equilibrium at the moving interfaces using the software THERMOCALC.

Rotation of Mo-fibers in directionally solidified Ni-Al-Mo eutectic alloys

The interface structure of α-Mo fib ers in directionally solidified Ni-Al-Mo eutectic alloys has been investigated by transmission electron microscopy and computer-aided graphical techniques. The Mo fibers have square or rectangular cross sections with well defined flat facets and fundamentally have the Bain orientation relationship with respect to the γ matrix lattice. The crystal lattices of the Mo fibers, however, are often rotated with respect to the matrix lattice about their longitudinal axes. Correspondingly, the facet planes deviate from the exact rational (1 1 0)γ′//(1 0 0)α relationship. A computer aided graphical plotting of the atom arrangement at the interfaces suggests that good atomic matching is kept even after the rotation of fibers. High resolution electron microscopy revealed the incorporation of single atomic structural ledges to keep the coherency of the lattices of the matrix and rotated fibers. The relation between the lattice rotation and the facet plane rotation is well explained by the moire and O-lattice models.

Interfacial structure and growth mechanisms of lath-shaped precipitates in Ni-45 wt% Cr

Abstract The interfacial structure of bcc laths precipitated from a nickel-rich fee matrix were studied using transmission electron microscopy and high-resolution electron microscopy (HREM). 50 precipitates and more than 200 defects in their vicinity were analysed. HREM images indicate the parallel conjugate planes of the Kurdjumov-Sachs orientation relationship are continuous across the broad face of the laths. In agreement with earlier studies, the (121)f habit plane contains structural ledges, irregularly spaced dislocations but no regular array of misfit dislocations. 87° of dislocations in the habit plane have Burgers vectors in the conjugate plane. These dislocations are found in the risers of growth ledges and must climb as the ledges migrate. The dislocations accommodate misfit in the conjugate planes of the orientation relationship while misfit perpendicular to these planes is compensated by elastic strain. Stacking faults found extending into the matrix from the precipitate habit plane accommodate elastic strain and appear to aid the formation of growth ledges. On the side facet of the precipitate lath, two types of linear defect are found: misfit dislocations spaced 12nm apart with ⅓[111]f Burgers vectors, and structural ledges parallel to the misfit dislocations spaced 1.5nm apart. The misfit dislocations are in a sessile orientation with respect to growth of the side facet.

Interfacial structure and growth mechanisms of lath-shaped precipitates in Ni-45 wt° Cr

Abstract The interfacial structure of bcc laths precipitated from a nickel-rich fee matrix were studied using transmission electron microscopy and high-resolution electron microscopy (HREM). 50 precipitates and more than 200 defects in their vicinity were analysed. HREM images indicate the parallel conjugate planes of the Kurdjumov-Sachs orientation relationship are continuous across the broad face of the laths. In agreement with earlier studies, the (121)f habit plane contains structural ledges, irregularly spaced dislocations but no regular array of misfit dislocations. 87° of dislocations in the habit plane have Burgers vectors in the conjugate plane. These dislocations are found in the risers of growth ledges and must climb as the ledges migrate. The dislocations accommodate misfit in the conjugate planes of the orientation relationship while misfit perpendicular to these planes is compensated by elastic strain. Stacking faults found extending into the matrix from the precipitate habit plane accommodate elastic strain and appear to aid the formation of growth ledges. On the side facet of the precipitate lath, two types of linear defect are found: misfit dislocations spaced 12nm apart with ⅓[111]f Burgers vectors, and structural ledges parallel to the misfit dislocations spaced 1.5nm apart. The misfit dislocations are in a sessile orientation with respect to growth of the side facet.

Interfaces In Quasicrystalline Systems

AbstractStructural relations between quasicrystalline and related crystalline rational approximant phases have been of interest for some time now. Such relations are now being used to understand interface structures. Interfaces between structural motif - wise related, but dissimilarly periodic phases are expected to show a degree of lattice match in certain directions. Our earlier studies in the Al-Cu-Fe system using the HREM technique has shown this to be true. The structural difference leads to well defined structural ledges in the interface between the icosahedral Al-Cu-Fe phase and the monoclinic Al13Fe4 type phase. In the present paper we report our results on the HREM study of interfaces in Al-Cu-Fe and Al-Pd-Mn systems. The emphasis will be on heterophase interfaces between quasiperiodic and periodic phases, where the two are structurally related. An attempt will be made to correlate the results with calculated lattice projections of the two structures on the grain boundary plane.

Mechanisms for the development of tent-shaped and invariant-plane-strain-type surface reliefs for plates formed during diffusional phase transformations

Tent-shaped and invariant-plane-strain type surface reliefs are considered for f.c.c.→ h.c.p., f.c.c.→b.c.c. and b.c.c.→ h.c.p. diffusional phase transformations that produce plate-shaped products. Mechanisms of growth by means of mobile structural ledges with disconnection character are described. Different mechanisms are proposed to account for the two types of surface relief in cases where the transformation product is a single crystal. The predicted relief shapes and relief angles are in good agreement with experimental observations.

Mobility of structural ledges

Mobility of structural ledges

TEM Characterization of Invariant Line Interfaces and Structural Ledges in a Mo-Si Alloy

Two distinct lath morphologies of Mo{sub 5}Si{sub 3} precipitates observed in MoSi{sub 2} differ in their cross-sectional shape and lattice orientation. Type I laths exhibit a rectangular cross section, with interfaces parallel to low-index planes, while Type II laths are parallelogram-shaped, with their major interface at 13{degree} to the Type I precipitate. The corresponding orientation relationships differ by a 1.8{degree} rotation around the lath axis. In this study, the difference between the two characteristic morphologies and orientation relationships is shown to be the formation of an invariant line strain for Type II precipitates. On an atomic scale, both interfaces have a terrace and ledge structure but differ in the stacking sequence of interfacial ledges associated with partial dislocations. The structural unit model and the invariant line model predict identical interface geometries which agree closely with the observations.

Hrtem Studies of Dislocations and Interfaces in TiAl

ABSTRACTIn this paper, we summarize the results of high resolution transmission electron microscopy investigations of the structure of dislocations and interphase boundaries in Ti-Al alloys. Dislocations of the type 1/2<110] and 1/2<112] have been examined in 60° and 90° orientations, respectively, in single-phase Ti-52 at%Al. The former dislocation exhibits an extremely compact core configuration, suggesting a high complex stacking fault energy, while the latter is dissociated the scale of several nanometers in a coupled intrinsic/extrinsic stacking fault configuration. Two types of α2/γ interface structures have been observed in nearly fully-lamellar Ti-48 at%Al. The first is consistent with the common observation of structural ledges due to 1/6<112> dislocations in the interface plane. The second interface type consists of a quasi-periodic array of 1/2<110] dislocations which do not lie in the interface, and appear to be contained wholy within the γ phase. The possible implications of these observations with respect to the mechanical behavior of both single-phase γ and two-phase lamellar microstructures are discussed.